Arthropod-borne members of the genus Borrelia cause relapsing fever and Lyme disease. We are taking biochemical and molecular biological approaches to the study of pathogenesis of these human disorders. With regard to the antigenic variation in relapsing fever, we have isolated genes of the proteins that antigenically vary during the course of infection. These genes were found as expression-linked copies and silent copies; the genes were located on linear plasmids. In our studies of Lyme disease, we used a bank of monoclonal antibodies and DNA probes and identified differences between strains. Recombinant DNA clones that express B. burgdorferi surface antigens are candidates for vaccines against Lyme disease.